So I've had the X4150 booted and running for about a day now. And it's managed getting on for 8 cpu-days worth of work so far.
It's looking good. On the workload we've tested so far, it seems to be about as fast as - if not marginally faster than - a comparable opteron, such as the X4200. Overall it can chew twice the workload because it has twice the cores.
It was a bit of a drag getting there, but I think it was worth the effort.